Dynamic loudspeakers achieve a particularly high efficiency, if the cross section of their moving coil is adjusted to the geometry of the membrane. Especially in case of small devices, such as mobile telephones, small loudspeakers with oval or rectangular shape are preferably used. There is a special requirement here for cost-effective manufacturing methods for moving coils with cross sections that are not circular, but of some other shape, especially predominantly rectangular or oval or elliptical.
For the manufacture of moving coils with a substantially rectangular cross section, the coil wire can be wound around square arbors and then held together with an applied adhesive. In order to achieve additional strength and dimensional control, it is necessary to resize the wound coil. In comparison with the manufacture of circular cylindrical coils, the working expenditure is about double, because circular cylindrical coils cannot only be wound faster, but they also have inherently higher strength, so no additional reworking is needed.
Producing coils with a rectangular or square cross section by reshaping a previously made circular cylindrical coil is already known. A method is known from the document JP 57 101 497 A, in which a circular cylindrical coil is put on a reshapable circular cylindrical coil support. A ram is then pressed into the inside of this coil support, which ram has a pointed head part with a pyramid-shaped base part projecting from the head part, which is in the shape of a prism. By pressing this ram, the coil support and the coil put on top are gradually reshaped from the circular cylindrical shape into a non-circular cylindrical shape with a square cross section.
A disadvantage of this known method is especially that a special coil support must be used, which is useless after the reshaping of the circular cylindrical coil and is therefore disposed of. The tapering edges of the pointed pyramid-shaped head part also cause shearing forces to occur which act on the coil through the coil support, which may cause damage to the coil.
Another method is known from the document JP 2002 135 891 A in which the reshaping of a circular cylindrical coil body is done by using a tool with a pointed head part and a base part, which tool is designed in a manner similar to the earlier described ram, where, however, the cross section of the tool, starting from a pointed circular conical head part gradually changes from a circular section into a rectangular section of the base part with a square shape.
This known method too causes undesired force components to act in axial direction, which force components can easily damage the coil to be reshaped or necessitate an appropriately massive coil design.